Flow and pressure will be measured in the microcirculation of skeletal muscle in preparations that exhibit vasomotor activity. Hydraulic pressure will be correlated to oncotic pressure as a function of time, and fluid balance will be calculated for the whole tissue on the basis of the parameters measured in the ensembles of capillaries which originate from different arterioles. The permeability of the venous microcirculation will be characterized in terms of the rate of extravasation of fluorescent molecules of varying molecular weight. These measurements will be done by a combination of low light level video techniques and video densitometry. These studies will be carried out in the microcirculation of the viscera and skeletal muscle and the data will be used to determine fluid balance in these tissues.